1. Field of the Invention
The present invention relates to a viscous fluid coupling which transmits the torque to a driven member, depending on the temperature conditions, more particularly, to a viscous fluid coupling suited to driving the cooling fan of an engine with appropriate torque by sensing the temperature in the engine room.
2. Description of the Prior Art
The structure of conventional viscous fluid couplings of the above type is such that is has an input member equipped with a rotor and an output member equipped with an internal chamber which houses a rotor and viscous fluid, said internal chamber being divided into a working chamber which accommodates a rotor and a reservoir chamber which stores fluid with a partition and the circulating passageway between the working chamber and the reservoir chamber being equipped with a valve device and a pumping mechanism. The valve device comprises a slot formed in the partition and a valve member which opens and closes said slot and which is driven by a temperature sensitive member. The temperature sensitive member senses the ambient temperature so as to increase the opening of the slot with an increase in the temperature by rotating the valve member.
Ordinarily, the shape of the slot is, as mentioned in the Japanese Patent Publication No. 27089 of 1965 (No. 40-27089), L-shaped or linear-shaped with a fixed width, and a plate member formed into a rectangular shape is used for the valve member. The relation between the slot and the valve member is such that the valve member rotates on the slot, so that the opening of the slot increases at a certain rate.
One may be apt to think that if the opening of the slot increases at a certain rate in proportion to the increase in the ambient temperature, the amount of the viscous fluid in the working chamber which contributes to torque transmission increases and the rotational speed of the output member also increases proportionally. As shown in FIG. 3, however, the rotational speed of the output member is not proportional to the amount of the viscous fluid in the working chamber, and a slight additional amount sharply increases the rotational speed of the output member and a saturated condition is reached immediately by a subsequent increase in the amount of the viscous liquid, so that the rate of speed increase shows a tendency to decline. Therefore, the increasing of the opening of the slot at a certain rate does not increase the rotational speed of the output member in proportion, as in the Japanese Patent Publication No. 27089 of 1965, and the rotational speed has two steps in a certain temperature region. Even when the shape of the slot is changed to a L-shaped one so that the opening becomes extremely large in the high temperature region, the rotational speed still has two steps and the curve is sharply raised in a temperature region as shown by broken lines in FIG. 4. When this conventional device is used to drive the cooling fan of the engine cooling system, the temperature region in which the rotation is changed over from low to high speed is normally set at a low level to protect the engine. That is to say, when the cooling capacity is not sufficient in low speed rotation, it is abruptly changed over to high speed rotation. This type of cooling causes overcooling of the engine for a while after rotation is changed over to high speed rotation, resulting in loss of the driving power, increased noise, and other disadvantages. It is also likely that the rotational speed of the fan changes periodically or so-called hunting takes place, depending on the relation.